This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted being prior art by the mere inclusion in this section.
Given the explosive growth of data traffic, the existing licensed spectrum is far from enough to provide satisfactory user experience and/or cost efficiency. In addition to the exclusive licensed spectrum, some shared, unlicensed spectrum may also be required. The 3rd Generation Partner Project (3GPP) has proposed to extend the Long Term Evolution (LTE) access technology to unlicensed spectrum. Licensed Assisted Access (LAA)-LTE (LAA-LTE) has been provided as one solution. The LAA-LTE utilizes an unlicensed spectrum as a performance booster managed by a licensed carrier. The traditional carrier aggregation framework used for licensed carriers can be reused in the LAA-LTE. A licensed LTE carrier is used for all mobility, control signaling and parts of the user data, while one or more carriers in an unlicensed spectrum are used to boost the user-data performance. For Release 13, DownLink (DL) data transmission in an unlicensed spectrum is supported and UL is also discussed. When coming to Release 14, UL data transmission in an unlicensed spectrum will be supported as well.
In the current cellular operation in an unlicensed band, e.g., in the LAA-LTE system, there are two steps for accomplishing UL data transmission. The first step is that a network device (e.g., a Base Station (BS)) makes a UL grant to let a terminal device (e.g., a User Equipment (UE)) know at which resource it should transmit data. In the second step, UE transmits UL data in the granted resource. Note that the same UL resource could be allocated to one or more UEs with a possible better efficiency when with listen-before-talk mechanism.
For the second step in the UL data transmission, Listen Before Talk (LBT) is done at UE side to avoid collision. In other words, UL transmission is subject to LBT success. Taking the LAA-LTE system as an example, the LBT operates as a MAC protocol in a radio node of LAA-LTE in the following way. First of all, the higher layer of UE firstly sends a data transmission request to its MAC layer. Before data is sent to physical layer for transmission over a wireless channel (e.g., a radio frequency band involving one or more continues or discontinues radio sub-bands or sub-carriers), the UE's receiver is activated to listen to the wireless channel. This may be referred to as a LBT success, if a radio frequency signal power listened over the wireless channel is less than a predetermined threshold. Otherwise, the receiver waits for a random time interval and then listens again. This receiver continues this listening after this random time interval, until the wireless channel is not occupied by other device or system. Once the UE's transmitter starts to transmit, it transmits the entire data frame without any stop even if collision occurs during the transmission.
FIG. 1 shows an example of UL LBT. As shown, the Channel Clearance Assessment (CCA) period (the left diagonal part) is used for UE to perform LBT. When a channel being sensed busy in the first UL granted subframe, i.e., subframe n in this example, UL data transmission does not take place for this subframe (denoted with a cross). In other words, the UL data transmission does not happen even when receiving UL grant at UE side. Then, when a channel is sensed as idle in the second sub-frame (e.g., subframe n+1 in this example), UL data is transmitted from UE side (denoted as the right diagonal part).
In a typical scenario involving the LAA-LTE and Wireless Fidelity (Wi-Fi) coexistence, it is possible that no UL data transmission happens even when an UL grant is sent from BS side.